(a) Field of the Invention
The present invention relates to an optical pickup apparatus capable of compensating for a thickness deviation of an optical recording media, and more particularly, to an optical pickup apparatus capable of compensating for a thickness deviation of the media, which is for near-field recording/reproducing, by using a solid immersion lens (SIL).
(b) Description of the Related Art
Recently, optical recording media such as optical and magneto-optical discs have been developed to have high-density storage, to be used for computer storage, package media for music and images, and so forth. In order to obtain high-density storage, it is desirable to reduce the light spot size of an optical pickup apparatus.
The light spot size is proportional to the wavelength of the light, and inversely proportional to the numerical aperture of an objective lens system. Therefore, many proposals to reduce the wavelength of the light as well as to increase the numerical aperture of the objective lens system have been suggested.
In order to increase the numeral aperture of the objective lens system, many optical pickup apparatuses have been developed using SILs which are nearly in contact with a signal recording surface of an optical disc. However, the SILs may collide with the optical disc, thereby allowing destruction of the signal recording surface due to rubbing or abrasion. Further, it is highly possible for the apparatus or the optical disc to suffer chemical or physical deformation due to high heat caused by light spot radiation.
In order to overcome the above drawbacks, optical pickup apparatuses that collect light from a signal recording surface through a substrate of an optical recording medium has been proposed in Japanese Laid-open Publication No. JP8-221790 and Korean Laid-open Publication No. 2002-0093287. In this type of optical pickup apparatus, since light is focused on the signal recording surface through the substrate of the optical disc by the SIL, it is necessary to compensate for thickness deviations of the substrate of the optical disc which likely result during the manufacturing process of the disc.
Thickness deviations of the optical disc are common. However, if a SIL is shifted for focus control in a pickup apparatus in which a light spot is focused by the SIL through a substrate of the optical disc, the optical energy changes such that near-field recording/reproducing becomes impossible.
The SIL cannot be shifted toward the signal recording surface when the optical disc has a thickness greater than a standard thickness. Further, the SIL cannot be shifted against the signal recording surface at a thinner area because an air gap must be properly maintained during near-field recording/reproducing. That is, since the thickness deviations of optical discs are relatively larger than the air gap of 100 nm, they cannot be compensated for by an adjustment of the air gap between the SIL and the disc.
Further, depth of focus is proportional to a wavelength of the light source, and inversely proportional to a square of the numerical aperture of a SIL or an objective lens system, which is defined as follows:Depth of Focus=λ/(2×NA2)where λ represents a wavelength of light, and NA represents a numerical aperture.
Therefore, when a numerical aperture of the objective lens system is 1.2 and a wavelength of the light is 650 nm, the depth of focus is a value of 225 nm, which is very short. Due to the very short depth of focus, the pickup apparatus may not approach the optical diffraction limit properly, with respect to the thickness deviation of the substrate.
Accordingly, focus controls are not accomplished by an adjustment of position of a SIL in near-field recording/reproducing, in principle, and the thickness deviation is not compensated for by adjustment of the air gap, especially in substrate-through recording/reproducing, because the SIL should always be nearly in contact with the disc.